In synchronous machines, voltage of several volts--ten-odd volts generates on the rotating shaft due to imbalance in magnetic flux or magnetomotive force, accumulation of charge derived from fluid in a turbine, grounding of the coil or the like. Such shaft voltage breaks down oil film on the shaft bearing and shaft current flows therethrough so that the bearing is damaged. Shaft voltage contains AC component and DC component, and the AC component involves mainly synchronous machine fundamental frequency (primary one: 50 Hz or 60 Hz) and synchronous machine tertiary harmonic wave (150 Hz or 180 Hz).
In the case where an excitation device of brushless excitation system type is employed, power source frequencies of the AC exciter appear as the AC component, while almost the same frequencies as that of the synchronous machine come out in the case where a thyristor type exciting device is used.
Heretofore, such a means in which a brush is allowed to contact with its shaft and a voltage across the shaft and the ground is determined has been known as a means for supervising shaft voltage of such type of synchronous machines. If a brushless turbine generator is taken as an example, the generator is constructed such that usually an earthing brush is mounted on its turbine side, and at the same time another brush is permitted to contact with a shaft of a brushless exciter on the opposite side of the turbine or a shaft of the turbine generator on the side of the brushless exciter to connect the brush with the ground through a resistance circuit, whereby a voltage value across both ends of the resistance circuit is detected to supervise abnormality in value of such shaft voltage.
However, in such supervisory unit as mentioned above, when the resistance circuit is short-circuited or when the brush becomes uncontacted with respect to the shaft, the shaft voltage cannot be detected and therefore, reliability of such supervisory unit is poor as the one for brushless synchronous machines.